Therapeutic radiation often is designed to penetrate deeply into a patient, as most targets for therapeutic radiation are located well below the surface. Even therapeutic radiation used to treat lesions on the skin surface still can penetrate several centimeters below the surface. To allow unaffected (normal) tissue to recover, the therapeutic radiation is generally delivered in many daily treatments, called fractions. Such fractional treatments rely on the principle that well-oxygenated normal tissues will repair and recover from radiation damage more quickly than the tissues being targeted by the therapeutic radiation. Even when the therapeutic radiation is delivered in fractions, permanent radiation damage to healthy tissues surrounding the radiation site could still occur.
It is therefore desirable to provide a radiation device that deposits a substantial portion of its radiation at the target tissues and much less radiation to the normal tissues surrounding the target. This is especially challenging for targets that are at or close to the surface. It is also desirable to avoid radiation exposure to healthy tissues that are adjacent to the target, irrespective of the depth of the target.
One example of the need for radiation devices capable of producing radiation doses at shallow depths is when treating scars and/or in ameliorating scar formation. Surgery inevitably produces scarring as a result of creating wounds. When wounds heal, there is an immediate inflammation of the wound site in which neutrophils infiltrate the wound which can cause excessive tissue loss in the scar area, leaving an area devoid of a matrix that is subsequently replaced with scar tissue through collagen synthesis and proliferation of other components in the extracellular matrix.
Radiation has been known to ameliorate this proliferation. However, in many conventional treatments radiation may not be provided until days or weeks after the surgery because it is logistically difficult to irradiate wounds in the operating room or the emergency room where most wounds are created. In scar irradiation, it is often desirable to irradiate to the depth of the dermis, sparing the epidermis as much as possible. The dermis varies in thickness and depth, depending on the anatomy. This requires a radiation device capable of irradiating at varying shallow depths of 1 mm to 10 mm with high precision, depending on the body location of the scar. For optimal scar treatment, an electron beam machine would have enough precision in order to achieve a range of penetration depth settings in increments of 2 mm or less, or even 1.5 mm or less, or even 1 mm or less increments.
Conventional electron beam machines, though, tend to produce beams whose tolerances can vary by +/−2 mm or more in terms of corresponding penetration depth. Such a large variation makes fine adjustments impossible, because the variation of the electron beam is as big as and is even larger than the desired tuning increments. Such a coarse variation means that conventional machines have a coarse precision with the result that electron beam energy adjustments are typically in relatively large increments of no less than 1 cm or even larger increments between penetration depth settings. The result is that conventional electron beam machines do not have as much precision as would be desired for improved scar treatments.
Another example is in vascular surgery, where repair of femoral and carotid artery blockages presents a high probability of restenosis. Irradiating the sutured junction of the repaired artery can prevent restenosis by inhibiting excess growth from the blood vessels walls. However, to be most effective, this radiation should be delivered at the time of the surgery, or shortly thereafter, to inhibit the excessive growth caused by the vessel repair while avoiding damage to tissues overlying or surrounding the blood vessels. Irradiation of a blood vessel may occur after anastomosis in some embodiments. Care desirably must be taken that the radiation is confined to the vessel walls and does not extend to nerves and tissues beneath the vessel.
Another example is in abdominal surgery, where low energy electron radiation may inhibit adhesion of surgically manipulated tissues, a common result of surgery. Adhesions can cause patient pain or discomfort and make re-operations at a later date more difficult.
Again, conventional electron beam machines suffer from a lack of precision to allow such finely focused therapies such as with respect to vascular or abdominal surgeries. This reinforces the strong desire to produce more stable electron beams to make it possible to achieve precise, shallow, penetration depths in fine increments.
X-ray radiation is another type of therapeutic radiation, X-ray radiation, though, is undesirable for these shallow depth therapeutic uses, as it penetrates deeply and can damage underlying tissues. Low energy x-rays (30 to 50 KV) have a limited penetration, but still could result in excessive dose delivered to the skin and epidermis. Electron beam radiation, therefore, is a better candidate for these shallow therapies than x-rays. Yet, there remains a need for new methods and devices for administering targeted electron beam radiation to patients in need thereof.
In principle energy can be controlled by means of chromatic magnetic elements in the beamline. One example is a dipole magnet system configured as a spectrometer. For a straight through beam, another approach is to look at “yield”, i.e., production of ionizing radiation per unit beam current. Use of a “yield servo” is known in the art for X-ray machines, which impact an electron beam on a bremsstrahlung conversion target to produce X-Rays. The intensity of X-Rays may be monitored by means of an ionization chamber downstream. Use of ion chambers for dosimetry is known in the art, both for reference dosimetry and machine dosimetry. See P. R. Almond, et al., The calibration and use of plane-parallel ionization chambers for dosimetty of electron beams, Med. Phys. 21, (8), August 1994 (“TG-39”); Raymond D. McIntyre, Transmission Ion Chamber, U.S. Pat. No. 3,852,610.
In the case of x-ray machines, the ratio of ionization chamber current collected to average beam current incident on a target provides an analog of energy, varying as roughly energy cubed. A yield servo for X-Rays is based on bremsstrahlung X-Ray yield from a target that destructively intercepts the electron beam. Bremsstrahlung is electromagnetic radiation produced by the acceleration or especially the deceleration of a charged particle after passing through the electric and magnetic fields of a nucleus. This approach works well in the x-ray context, because radiative yield varies as energy cubed. With the large currents (e.g., 100 mA) typical of x-ray machines, signals are robust. However, it is not possible to use such a conversion target for an electron beam, as the beam would be destroyed. In fact, a requirement on beam monitors and other beam line devices for a straight-through electron machine is to avoid bremsstrahlung contamination in the treatment field. This poses a technical challenge of how to monitor the electron beam energy without interfering with beam quality in a straight-through electron machine.
Electron beam machines, as a result, are different than x-ray machines and principles for highly precise control of electron beam settings and fine adjustment to other settings is not yet known in the field of shallow therapeutic treatments. For electron beam machines of variable energy, two approaches are used in the field today to control electron beam energy. One is the spectrometer-based approach, where dispersion is generated to spread the beam out in a radial dimension and pass the beam through water-cooled slits of width corresponding to 6% in energy. The beam is then recombined by the magnetic optics and, directed toward the target volume. This approach generates background radiation at the slits, and other parts of the bend magnet system. This necessitates additional shielding, additional strengthening of mechanical members to hold the shielding, and, altogether a 17,000 lb machine that is not mobile and requires a specially designed vault with concrete walls that are several feet thick. The other approach is the straight-through electron beam machine, as with the MOBETRON unit commercially available from IntraOp Medical Corporation. Straight-through electron beam systems are designed to produce very low stray radiation and thus can operate safely in unshielded environments. Examples of such machines are described in U.S. Pat. Nos. 5,321,271 and 5,418,372. Instead, system parameters are tightly specified and deviation from an acceptance range results in interlock and beam off.
The MOBETRON system stably operates at three energies 6, 9 and 12 MeV without closed loop feedback. Energy stability is maintained through special rf-circuitry or through other means such as is described in U.S. Pat. No. 5,661,377, but there is no energy servo to control and modulate the electron beam other than to set the beam energy at these three levels Instead, system parameters are tightly specified and deviation from an acceptance range results in interlock and beam off.
There is a need for a radiation device that is capable of delivering more stable, electron beam radiation with higher precision to many shallow depths in fine increments with little radiation exposure to both nearby tissues and tissues below the target. In order to control radiation to accurately penetrate to shallow depths and to allow the radiation to be adjusted to other depth settings in very small or even continuous increments, improved strategies to stabilize and control the penetration depth of the electron beam are needed.